1. Field of the Invention
The invention relates to vehicle brakes and in particular to brakes having camming levers for transmitting the braking actuating force to the brake shoes. More particularly, the invention relates to a brake construction having such camming levers in combination with a hydraulic service brake actuator and a mechanical actuated parking brake mechanism both of which operatively engage the levers for expanding the brake shoes outwardly into engagement with a brake drum, and in which the parking brake mechanism is unaffected by the operation of the hydraulic service brake and vice versa.
2. Description of the Prior Art
Numerous brake constructions have been developed to increase and transmit efficiently the actuating force of a hydraulic cylinder to the brake shoes for expanding the brake shoes outwardly into braking engagement with a brake drum. Such constructions use various anchored and moveably mounted brake shoe combinations to increase the brake shoe efficiency and to provide a satifactory low-cost brake construction.
In many brake constructions the hydraulic actuating force is applied to the brake shoes by direct engagement of the fluid actuated pistons with the brake shoe ends or a component thereof. Many of these constructions use a straight hydraulic cylinder arrangement in which a pair of pistons move outwardly in opposite generally horizontal directions along the axis of the cylinder. The outward force exerted on the shoes by the pistons has a negligible vertical component. Normal braking force component is the result of this horizontal component. The addition of a small downward component of the actuating force provides significant increase in the normal force and consequently the brake torque which is proportional to the normal force.
V-shaped hydraulic cylinders such as shown in U.S. Pat. Nos. 3,322,237 and 3,482,658, apply the actuating force in a downwardly angularly direction against the brake shoe or actuating components as opposed to the horizontal direction of usual straight cylinder pistons. This angularly applied force increases the normal component of the actuating force, which increases significantly the normal force component of the braking torque which correspondingly increases the braking torque between the drum and brake shoes. Such V-shaped or angled actuating cylinders, however, are more expensive than the usual straight cylinders, resulting in an undesirable cost increase of the resulting brake in order to achieve the increased or self-energizing braking action.
One known brake construction, shown in U.S. Pat. No. 2,372,322, uses a straight cylinder, double-piston fluid actuator in which the actuating forces are applied to the ends of levers, which in turn exert a spreading force to pairs of brake shoes at their anchored abutting ends. Such a construction does not provide the sliding camming action of the improved brake construction as set forth below, nor is such a construction intended to be used in combination with slidably mounted or floating brake shoe arrangements.
It is desirable in many vehicle brake constructions to provide both a hydraulically actuated service brake mechanism and a mechanically actuated parking brake mechanism. Examples of such dual brake constructions are shown in U.S. Pat. Nos. 2,060,874, 2,127,739, 2,374,526, and 3,023,853. Preferably, such dual brake constructions use many of the brake components for expanding the brake shoes outwardly into braking engagement with the drum upon actuation of either the service or parking brake mechanisms. It also is desirable to have the remaining brake components which are used specifically for either the service or parking brake mechanisms to be unaffected upon actuation of the other braking action, thereby eliminating unnecessary wear of the components and needless mechanical movements.
No brake construction of which we are aware provides a pair of camming levers in combination with a straight cylinder dual-piston hydraulic actuator and a pair of floating brake shoes to achieve the advantages of a V-shaped cylinder, by increasing the vertical or normal component of the piston's horizontal actuating force through a sliding camming action between the levers and brake shoes which results in a significant increase in braking torque. Likewise, no known camming lever actuated brake construction is provided with a mechanical parking brake mechanism operatively connected to the levers by links having a floating or freely moveable connection point.